A photo-sensor, i.e., a storage type photo-sensor having functions to store and read out an electric signal according to the quantity of an incident light is advantageous over a non-storage type photo-sensor in that it can produce a more intense signal to attain a higher S/N ratio because it stores an electric signal generated by an incident light during its storage time. Storage type photo-sensors and signal reading methods are known. A system for reading signals by scanning an electron beam a photoconductive type image pickup tube (the operating principle of which is disclosed in Japanese Patent Laid-Open No. 58-194231) has been disclosed in the prior art, as have a system for reading out signals through a switching element disposed at each picture element and its wiring a solid-state image sensor (as referred to pp. 123 to 134 of "Imaging Technology" published by Corona Corp.) and a system for reading out signals by irradiations with a reading light for reading out the signals (as referred to pp. 81 to 87 of SPIE, Vol. 173 (1979)).
In the prior art, on the other hand, electrophotography using a photoconductive material has a wide variety of uses including in a facsimile machine and in a copying machine. In the medical field, moreover, electrophotography called Zeroradiography is used for dental or breast inspections. In this medical field, the digitization of an image has recently advanced as far as the desire to develop a system capable of not forming an analog image using toner like the aforementioned Zeroradiography but reading out the image in the form of electric signals and digitizing the signals. The papers having proposed a system of that kind are exemplified by Japanese Patent Laid-Open No. 59-99300, U.S. Pat. No. 4,268,750 and pp. 176 to 184 of SPIE, Vol. 626 (1986).
These photo-sensors adopt various signal reading methods and the structures therefor. One example of the conventional device using no signal reading light will be described below.
With reference to FIG. 17, an X-ray imaging device will be described as an example of the photo-sensor using a storage type photoconductive layer. FIG. 17 is a diagram showing a fundamental structure for explaining the operating principle of the photo-sensor. In this photoconductive device, charges are stored by a corona discharge in the surface region of a photoconductor 1701 formed over a substrate 1702, and the photoconductor 1701 is then irradiated with an X-ray. As a result, electron-hole pairs are generated by the interaction between the X-ray and the photoconductor 1701 and are moved by the internal electric field of the photoconductor 1701 to neutralize the charges which are stored in advance in the surface of the photoconductor 1701. At this time, a charge pattern according to the intensity distribution of the incident X-ray is formed in the surface of the photoconductor 1701. Since, this charge pattern is left in the surface of the photoconductor as the residual charge, i.e., the residue of the charge neutralized by the incident X-ray, it produces an X-ray image when it is read out by an electrometer 1703. This electrometer 1703 is equipped with a plurality of probes, which can scan the vicinity of the surface of the photoconductor 1701 to detect the potential pattern. The residual charge Q is expressed by the following equation if the charge stored by the corona discharge is designated at Q.sub.O and if that component of the charge generated by the X-ray irradiation, which has reached the surface of the photoconductor 1701, is designated at Q.sub.S : EQU Q=Q.sub.O -Q.sub.S - - - ( 1).
Here, the component Q.sub.S is less than the generated charge because of the recombination of the electrons and holes in the photoconductor 1701. Considering this recombination effect, a carrier generation efficiency n is introduced, as will be defined by the following equation (2): EQU =d.xi./dn E - - - (2).
This efficiency is known to take a substantially constant value. Here, the term d.xi./dn is defined as an energy necessary for generating one charge in the surface of the photoconductor 1701, and the letter E designates the electric field in the photoconductor 1701. Those values .xi., n and E are given by the following equations: EQU .xi.=f k X A EQU n=Q s A/e; and EQU E=Q s/C d - - - (3).
Here: letter f designates the energy absorption efficiency for the radiation of the photoconductor; letters k and X designate the energy flux per unit exposure of the incident radiation and the exposure itself, respectively; letters A, d and C designate the area, the layer thickness and the electrostatic capacity per unit area of the photoconductor, respectively; and letter e designates an elementary electric charge.
From these, the relation between the signal charge and the exposure is obtained by the following equation: ##EQU1##
Incidentally, this equation is simplified in a low exposure region, as following: ##EQU2##
Here, letter E.sub.O designates an electric field to be applied to the photoconductor before the irradiation with the X-rays.
Like this example, the device for directly reading out the pattern of the signal (or charge), which is stored in the photoconductor, as a potential variation with the electrometer has its potential reading electrode and its scanning structure complicated and enlarged. Of the photo-sensors, moreover, the conventional sensor for scanning with an electron beam like the image pickup tube is required to have a casing structure to be evacuated. In the solid-state imaging element of the prior art on the other hand, a switching element for reading out signals has to be disposed in each picture element and arranged in a high density.
On the contrary, the photo-sensor of the type for optically reading out the stored charges is effective for solving the above-specified various problems. The conventional structure of the optical reading type photo-sensor and the operating method thereof will be described below in connection with its fundamental principle.
FIGS. 3A and 3B are diagrams showing one example of the fundamental structure of the optically reading type photo-sensor of the prior art and explaining the operations thereof, respectively. Reference numerals 301 and 304 designated transparent electrodes; numeral 302 a photoconductor; and numeral 303 an insulator. FIG. 3B illustrates the electric potential distribution in the photo-sensor at the individual steps of the operation to be described in the following. The abscissa designates the distance x normal to the light receiving plane, and the ordinate designates the electric potential V. First of all, a DC source 305 having a voltage V.sub.1 is connected between the transparent electrodes 301 and 304. If the photoconductor 302 and the insulator 303 have electrostatic capacities C.sub.1 and C.sub.2, the potential distribution in the photo-sensor takes a form, as indicated at 310 in FIG. 3B. If, in this state, the photoconductor 302 is optically irradiated, the generated electrons are stored in the interface 315 with the insulator 303 so that the potential distribution reaches an equilibrium, as indicated at 311. Next, a switch 306 is turned to connect the photo-sensor with a DC source 307 having the opposite polarity of the DC source 305. At this time, the potential distribution is indicated at 312 to establish again the electric field in the photoconductor 302. This is the ready state of the example of the prior art. If, in this state, a light 320 to be observed (which will be called the "signal light") comes from the outside, the resultant holes are moved to the interface 315 between the photoconductor 302 and the insulator 303 by the internal electric field of the photoconductor 302 generated by the DC source 307 until they are recombined with the electrons stored in advance in that interface 315. As a result, the potential distribution takes a form, as indicated at 313. If the quantity of charge generated by the signal light 320 is designated at Q.sub.S, the potential at the aforementioned interface 315 varies by Q.sub.S /(C.sub.1 +C.sub.2). After the signal light 320 has been shaded, the switch 306 is turned again to connect the transparent electrode 301 with the DC source 305. As a result, the potential distribution takes a form, as indicated at 314. If a reading light 321 for signal reading is then caused to irradiate the photoconductor 302, the potential distribution restores the equilibrium 311. In this meanwhile, the charges C.sub.2 Q.sub.S /(C.sub.1 +C.sub.2) proportional to the signal charges Q.sub.S generated by the signal light 320 flow through a load 308 and are detected at an output terminal 309. Incidentally, in case the polarities of the DC sources 305 and 307 are opposite to the aforementioned ones of FIGS. 3A and 3B, the polarities of the charges to be stored and recombined and the potentials are inverted, the operations are similar on principle so that their detailed description will be omitted.
FIGS. 4A and 4B are diagrams showing a structure based on another principle according to the prior art and the potential distributions in four states of the operations, respectively. In FIG. 4B, the abscissa designates the distance x normal to the light receiving plane, and the ordinate designates the potential V. This photo-sensor is constructed of a lamination of a transparent electrode 401, a photoconductor 402, a charge storage layer 415, a photoconductor 403 and a transparent electrode 404. The following description is directed to the operational principle in case the charge storage layer 415 has a function to store electrons. A DC source 405 is connected with the electrode 401 to apply a negative voltage of -V.sub.1 to the laminated structure (between the electrodes 401 and 405). The potential distribution in this state takes a form, as indicated at 410 in FIG. 4B, if the electrostatic capacities of the photoconductors 402 and 403 are designated at C.sub.1 and C.sub.2. If a signal light 420 is then incident upon the photoconductor 402, the electron-hole pairs are established in the photoconductor 402 so that the electrons migrate toward the charge storage layer 415 until they are stored therein, as indicated at a potential distribution 411. If, at this time, the quantity of charges generated by the signal light 420 is Q.sub.S, the potential of the charge storage layer 415 drops by Q.sub.S /(C.sub.1 +C.sub.2). If the switch 406 is turned to drop the potential of the transparent electrode 401 to zero after the signal light 420 has been shaded, the potential distribution takes a form, as indicated at 412. If the photoconductor 403 is then irradiated with a reading light 421 for signal reading, the electron-hole pairs are generated so that the holes migrate to the charge storage layer 415 to neutralize the electrons previously stored, until an equilibrium is reached, as indicated at a potential distribution 413. Meanwhile, the quantity of charge C.sub.1 Q.sub.S /(C.sub.1 +C.sub.2) proportional to the signal charge Q.sub.S flows through a load 408 so that it is detected as a voltage signal from an output terminal 409.